Understanding And Performing Standby Power Measurements

Measurement of standby power is important to the engineer designing, testing and certifying power supplies and everyday domestic and office appliances. While the amount of power used per device is low, standby power consumption can quickly add up. Power analyzers, together with test automation software, provide flexible and powerful tools to measure standby power.

Jon Francis, Product Marketing Manger, Tektronix | Aug 20, 2014

Standby power refers to the electric power consumed by electronic appliances while they are switched off or in a standby mode – but still connected and still drawing power. Fundamentally, standby power is the power used while an electrical device is in its lowest possible power mode.

Devices such as TVs or microwave ovens may be inactive but are always ready to work when in standby mode, showing the time on a digital clock or waiting for input from a remote control. Other devices, such as power adapters for laptop computers, tablets and phones consume power without offering any visible features when in standby mode. These and many other devices all consume standby power.

The standby power of household electronic devices is typically very small, but the sum of all these devices within the household can easily become significant. For both home and business, standby power makes up a notable portion of the steadily rising miscellaneous electric load, which includes small appliances, security systems, and countless other small power draws.

For example a typical microwave oven consumes more electricity powering its digital clock than it does heating food. For while heating food requires more than 100 times as much power as running the clock, most microwave ovens stand idle—in standby mode—more than 99 percent of the time.

Standby power is typically 1 or 2 Watts for a household appliance and even less for computing devices. Despite the low power draw, the fact that the devices are continuously plugged in along with the sheer number of such devices in the typical household or business means that standby power usage can reach up to 22 percent of all appliance consumption, and 5 to 10 percent of total residential consumption.

Put it all together and standby power costs add up. On a personal level, standby power adds about $100 annually per household in the United States. This in turns requires extra electricity generation and transmission infrastructure while making it that much more difficult for nations to achieve energy independence. What’s more, an estimated 1 percent of global CO2 emissions are due to standby power.

A number of government-mandated programs have been put in place over the years in an effort to reduce standby power including ENERGY STAR and the EU Eco Directive. The scope of these programs continues to grow and the level of standby power in Watts necessary to achieve compliance steadily falls. For example the European Community mobile device charger 5-star rating requires a standby power consumption of less than 30 mW.

Standby Measurement Challenges

As standby power increasingly comes under regulatory scrutiny, designers at all levels must find ways to minimize standby power consumption. Determining whether design changes and enhancements are actually helping the cause requires the ability to make accurate and repeatable measurements. In addition, pre-compliance testing is advisable to ensure that designs will pass formal compliance tests and meet government and industry requirements.

Standby power is measured with a wattmeter or power analyzer. However, accurately determining standby power usage involves more than a simple measurement of power in watts. Key challenges, as detailed below, include:

· Low power and current.

· Highly distorted waveforms since power supplies operating at low load often draw very high crest-factor current.

· A low power factor because the current may be predominantly capacitive, through the power supply EMC filter.

· When the power supply is in burst mode, drawing power irregularly rather than continuously.

Let’s take a closer look at each of these challenges.

Measuring Low Power and Current

Since low power is the nature of the game for standby power measurement, not all wattmeters or power analyzer are equipped for the task. In particular, the power analyzer must have a suitable range for measuring the current. In general, measurements below 5 percent of a current range will not be reliable. Here is an example of how this is calculated:

Measure 100 mW at 230V and power factor =1.

Watts = Volts x Amps x PF

So

Amps = Watts / (Volts x PF)

= 0.1 / (230 x 1) = 0.4 mA

This means that the power analyzer should operate on a range that is 2 mA or lower.

High Crest Factor Waveforms

At low load, the current is often at its most distorted. Current is drawn only at the peak of the voltage to charge the power supply’s reservoir capacitors and appears as a short pulse. Crest factor is a function of peak value over RMS value.

The power analyzer must be able to measure without clipping or reduction in accuracy when the crest factor is greater than 3, possibly up to 10.

Under standby conditions the input current may be dominated by current in the capacitors used for EMC filtering, especially the X2 rated capacitors fitted across line and neutral. In this case, the current is phase shifted by up to 90 degrees. This is an area where not all power analyzers will measure accurately.

Operating at no or low load the power supply’s own control and power circuits still operate to maintain a regulated output voltage. This control power may be in excess of the desired standby power so many power supplies switch to a burst mode.

In this mode, the power switching devices inside the power supply stop operating and the output voltage is maintained exclusively by the output smoothing capacitors. When the output voltage falls to a pre-determined level then the power supply switching starts again to top up the output capacitors.

In this mode, current is drawn in bursts from the AC line. The bursts of current are irregular and vary in duration and size. In addition, the power drawn by the product under test may simply change due to temperature or further power saving features.

For measurements of power in this circumstance the power analyzer must:

· Sample power continuously so as not to miss any power.

· Take an average of the power over a period of time long enough to provide a stable result.

Making Connections

Once you’re confident that you have a suitable instrument in hand, the first step in any measurement exercise is making the appropriate connections to the device under test and the power analyzer. Here are few tips to keep in mind:

· The power analyzer will sample the voltage and current waveforms simultaneously in order to calculate power. Connections should be safe and secure.

· Voltage is measured by connecting the voltage terminals in parallel.

· Current is measured by connecting current terminals in series. In general, a directly connected resistive current shunt (as opposed to a current transformer) is required to achieve reasonable measurements.

· For standby power measurements the voltage connection is made to the source or supply side of the circuit.

· During normal power measurements the current and power in the voltmeter circuit of the power analyzer is considerably less than that of the current shunt.

Fig. 1. Connection for measuring power in normal mode.

· For standby power measurements the current and power in the voltmeter circuit can be significant and that of the current shunt very small. So for standby measurements the voltage is connected on the supply side of the current shunt. The difference in connections are illustrated in Figs. 1 and 2.

Fig. 2. Connections for measuring power in standby mode.

Basic Measurements

For basic measurements, set up your power analyzer to measure Watts. Conditions should be:

· Continuous sampling

· Recording Watts at a rate faster than 1 a second

· Averaging over a selectable time.

Most advanced power analyzers can be used to make basic standby measurements by auto-ranging for crest factors up to 10, sampling continuously at 1MHz without gaps and averaging the power measurement (watts) over at set period of time. A basic measurement like this is useful for continuous use in everyday product design and development.

Understanding Compliance Measurements

Power analyzers are used to make measurements to check compliance against standards such as IEC62301 Ed.2:2011 and EN50564:2011. IEC62301 Ed2 is important because this is the definitive measurement method reference by ENERGY STAR and the European regulation No 1275/2008 standby and off mode electric power consumption of electrical and electronic household and office equipment.

Before getting started with testing for regulatory compliance, it’s helpful to become acquainted with the requirements. The most important considerations are listed below. As always, be sure to refer to the latest edition of the standard and confirm details before making compliance measurements.

Supply Voltage Measurement

· Nominal voltage and frequency for the region may be used but must be stable +/- 1 percent.

· The total harmonic content (THC) must not exceed 2 percent. (THC is a modified THD or total harmonic distortion including the first 13 harmonics only).

· Crest factor of the voltage (ratio of peak to RMS) must be between 1.34 and 1.49.

· Since any variation in these parameters will influence the standby power measurement they must be measured and confirmed simultaneously with each power measurement during the test. Simultaneous RMS and harmonic measurements are required.

· The normal AC line supply may meet these criteria, especially if the test connection is made close to the incoming supply or distribution transformer. If the supply does not meet the requirements than a synthetic AC source or line conditioner must be used.

Measurement Uncertainty

The IEC standard takes into account the difficulties discussed above and defines a measurement uncertainty that is based on both the level of power to be measured and the distortion and phase shift of the waveform.

To take into account both distortion and phase shift, Maximum Current Ratio (MCR) is defined as:

MCR = Crest Factor / Power Factor

The required level of uncertainty, “U” can then be determined as shown in the power measurements flowchart in Fig. 3.

There are three possible methods for determining the power in Watts as defined in the standards:

1. Direct Method -- This is the basic power analyzer front panel method described earlier. It is intended for rapid prototyping measurements on products that draw very stable power only.

2. Average Reading Method -- This method is a modified version of that used by previous versions of the standard. Since the measurement takes a minimum 20 minutes and does not apply to all product modes, the sampling method described below is preferred.

3. Sampling Method -- This is the method recommended by the IEC. It is the fastest and applies to all possible product modes. Here are a number of factors to be aware of when performing tests using this method:

· Power and other measurements are recorded at a rate faster than 1 per second.

· The product under test is energized for a minimum 15 minutes

· The first third of the data (5 minutes) is discarded

· Stability of the measurement is determined from a least-squares linear regression through all the power measurements. Stability is established when the slope of the straight-line regression is either less than 10mW/h (input power <= 1W) or less than 1 percent of the power if the power is greater than 1W. An example of this is shown in Fig. 4.

Fig. 4. This example shows a power measurements and least-squares stability check.

Power Analyzer Requirements

The IEC62301 Ed.2 regulation also outlines general power analyzer requirements. The most important include the following:

1. Ability to determine all measurements (Volts, Amps, crest factors, THC as well as Watts) and record them simultaneously at an interval of less than 1 second.

2. Sample continuously without gaps.

3. Have a power resolution of 1mW or better

4. Rated measurements at crest factor of 3, preferably 10

5. Minimum current range of less than 10mA

6. Signal over-range

7. Ability to switch auto-ranging off.

8. Frequency response of at least 2 kHz

Making a Compliant Standby Power Measurement

Performing standby power measurements involves first pulling together the necessary equipment. For certification, a programmable AC power source that allows various voltage and frequency combinations is required along with a power analyzer that meets the requirements of IEC62310 Ed.2 for uncertainty, measurement procedure and general characteristics as described above.

In addition, you need a means to connect the test circuit (AC supply, power analyzer and product under test) safely and in accordance with IEC62301 Ed.2 Section B.4. A high quality breakout box is the easiest way to satisfy this requirement. Lastly, you’ll need a way to record and report the measurement results.

Fig. 5. Results from standby power testing can be printed as a PDF or exported to a spreadsheet for further analysis.

Test instrumentations vendors typically offer software support and compliance wizards that can confirm test set-up and automate many of the mandated tests. Note that test duration should be a minimum of 15 minutes and longer if requirements for stability are not met. Results are then captured for reporting at the end of the test. Fig. 5 shows the results of a standby compliance test run.